The present invention relates to an improved intrusion detection system and more particularly to an intrusion detection system having a plurality of different types of sensors wherein one of the sensors is more susceptible to false alarm than the other. The intrusion detection system of the present invention provides for improved immunity to false alarm.
Intrusion detection systems having a plurality of detectors, detecting an intrusion as evidenced by different physical phenomenon, to improve immunity to false alarm is well-known in the art. Typically, for example, an intrusion detection system having a plurality of sensors comprises a passive infrared sensor, to detect infrared radiation, directed to detect intrusion in a volume of space and a microwave detector, to detect mass motion, directed to detect intrusion in the same volume of space. When a signal is generated by both of the sensors, the signal processing circuitry gates the signals and generates an alarm signal.
Another example of an intrusion detection system employing a plurality of sensors is disclosed in U.S. Pat. No. 4,853,677 (See also U.S. Pat. No. 4,928,085). There, a single microphone detects the audible sound of glass breaking and the subsonic sound of pressure on the glass being flexed. Here again, although a single microphone is used, two different types of physical phenomena are detected (audible sound waves and low frequency pressure waves) in order to provide the detection system with greater immunity to false alarm.
In all the intrusion detection systems of the prior art employing a plurality of sensors, the signal from each of the sensors is held for a period of time before they are brought together to be compared to determine if an alarm condition has occurred. This is because the detection of an intrusion causes a momentary pulse signal and the presence of the intrusion on both of the sensors may not occur simultaneously. Thus, it is desired in the prior art to "stretch" or hold each of the pulses received by each of the sensors.
While this has worked generally satisfactorily, the incidence of false alarm is still too high to be tolerated. False alarm can arise from the following example. Since the sensors detect different physical phenomena, each sensor is prone to generate false alarm differently from one another. Thus, for example, in a combination of passive infrared sensor with a microwave sensor, a microwave sensor can detect motions beyond a confined space contained by glass or sheetrock. Thus, a microwave sensor can extend beyond the desired coverage area, making it more susceptible to the generation of false alarm.
When one of the sensors of a multiple sensor intrusion detection system is more susceptible to false alarm than the other, i.e. the sensor generates more detected sensing pulses than the other, the immunity to false alarms of the entire system is reduced. This occurs because if, for example, one of the sensors, the one that is more susceptible to false alarm, is frequently generating alarm signals, then because the signal is being held by the holding circuitry, the time period in which that sensor is on is virtually a constant. Then, if another type of physical activity triggers a false alarm in the second sensor, this would create a false alarm in the entire system.
Accordingly, in the present invention, an improved intrusion detection system having a plurality of sensors which is more immune to false alarm generation is disclosed.
U.S. Pat. No. 4,882,567, discloses an intrusion detection system having a plurality of sensors. The reference discloses the use of one type of sensor to detect an intrusion then activating the second type of sensor. The purpose of this device is to decrease power consumption in a multi-sensor detection system. Because the second inactive sensor is not on, the detection signal from the first sensor must be held for an inordinate amount of time (on the order of more than ten seconds) in order to insure that by the time the inactive sensor is activated, the signal from the activating sensor would still be held to be compared to the signal from the activated sensor.